Position sensor having core with high permeability material

ABSTRACT

A position sensor for a medical device comprises a core made of a high permeable material such as Wiegand effect material comprising a mixture of cobalt, vanadium, and iron. The position sensor has an outer diameter of approximately 0.4 mm and is used in a medical device having an outer diameter of approximately 0.67 mm.

FIELD OF THE INVENTION

[0001] The present invention relates generally to object trackingsystems, and specifically to position sensors having high sensitivity athigh temperatures for tracking the position and orientation of a medicaldevice.

BACKGROUND OF THE INVENTION

[0002] In many medical procedures, devices, such as probes, endoscopes,catheters, stents and tags/markers are inserted into a patient's body.Such devices are used for a large variety of procedures includingirreversible surgical actions, such as ablation and taking of tissuesamples. Therefore, it is necessary to have accurate information on theposition and orientation of the probe within the patient's body.

[0003] Electromagnetic position determining systems provide a convenientmethod of receiving accurate information on the position and orientationof intra-body objects, and allow accurate tracking of these objects.Such systems are described for example in U.S. Pat. Nos. 5,558,091,5,391,199 and 5,443,489, and in International Patent PublicationsWO94/04938 and WO96/05768, whose disclosures are incorporated herein byreference. These systems determine the coordinates of a device using oneor more field sensors, such as a Hall effect device, coils or otherantennas carried on the device. The field sensors are transducers usedas position sensors and are typically located at or adjacent the distalend of the device, and/or along the length of the device. Therefore, thetransducers are preferably made as small as possible so as to fit intothe device without interfering with the device's maneuverability orincreasing its size unduly.

[0004] U.S. Pat. No. 5,558,091 describes a Hall effect sensor assemblyof a cube shape which includes three mutually orthogonal, thingalvanomagnetic films. This sensor assembly is preferably of dimensionsabout 3×0.75×0.75 mm. The U.S. Pat. No. 5,558,091 further describesanother Hall effect sensor assembly which includes three field sensingelements in the form of semiconductor chips. Each chip includes one ormore elongated bars of a magnetoresistive material. Each such chip issensitive to magnetic field components in the direction of the bar. Thisassembly preferably has a diameter of 0.8 mm or less. However, suchchips suffer from nonlinearities, saturation effects, hysteresis andtemperature drifts.

[0005] Therefore, most magnetic position determining systems use sensorsformed of miniature coils that include a large number of turns of anelectrically conducting wire. Such coils are described, for example, inPCT publications PCT/GB93/01736, WO94/04938 and WO96/05768, in the abovementioned U.S. Pat. No. 5,391,199, and in PCT publicationPCT/IL97/00009, which is assigned to the assignee of the presentapplication, all of which are incorporated herein by reference. Theperformance of a sensor coil is dependent on its inductance, which is afunction of the number of turns of the coil times the cross sectionalarea of the coil. Therefore, in planning and designing a miniature coilfor use within a surgical device, for example, it is generally necessaryto make a compromise between performance and the size of the coil. Suchcoils are typically used in position sensors having three mutuallyorthogonal sensor coils and typically have minimum dimensions of0.6×0.6×0.6 mm and more generally of 0.8×0.8×0.8 mm. It has always beenbelieved that smaller coils of the same type would not provideacceptable performance and are also hard to manufacture. Additionally,given these fixed size limitations, no sensor coils have been developedthat have an outer diameter less than 0.6 mm.

[0006] Moreover, for these types of position sensors, it is common forthe sensor coil to include a core. For those position sensors (sensorcoil) that utilize a core, it is known that the material for the corecan consist of two acceptable materials. The first material is ferriteand has been used with success as a core material for medical deviceshaving a sensor coil with a core.

[0007] The later core material developed that has also proved to beeffective as core material for a sensor coil in a medical device iscarbonyl iron. However, for both types of core materials, the sensorcoils utilizing such core material would be generally limited to theouter diameter minimum dimension requirements described above.

[0008] In order to determine both translational and rotationalcoordinates, some position determining systems, such as the systemdescribed in the above-mentioned PCT publication WO96/05768, use threesensor coils, having respective axes that are mutually linearlyindependent, preferably mutually orthogonal. Preferably, these threecoils are packaged together to form a sensor assembly, which is used toprovide six-dimensional position and orientation coordinate readings.The use of an assembly which has the three coils within one packageallows easy insertion and/or attachment of the coils to devices such ascatheters. Also, the assembly provides exact positioning of the coilsrelative to each other, thus simplifying the calibration of positiondetermining systems using the coils. Generally, the coils are enclosedin a cylindrical-shaped case, which protects the coils from thesurroundings.

[0009] In the system of the '768 publication, this assembly typicallyhas a length of about 6 mm and a diameter of about 1.3 mm. Because theaxes of the coils need to be generally mutually orthogonal in order toachieve accurate position sensing in all six dimensions, it is notpossible to make the diameter of the assembly much smaller.

[0010] Although this coil assembly fits into most medical devices, insome cases coils of equivalent performance and smaller width aredesired. For example, U.S. Pat. No. 6,203,493, which is assigned to theassignee of the present invention and is incorporated herein byreference, describes a method of enhancing the accuracy of positiondetermination of an endoscope that includes miniature position sensingcoils, by distancing the coils from metallic apparatus within theendoscope. If the coil assembly can be made with a smaller width, it isthen possible to increase the separation between the miniature coils andthe metallic apparatus, and thus achieve better accuracy from theposition determining system. Coils made by photolithography or VLSIprocedures are known as disclosed in U.S. Pat. No. 6,201,387 BI, whichdisclosure is incorporated herein by reference, in which these coils arereferred to as photolithographic coils. Photolithographic coils aregenerally made in the form of a spiral conductor printed on a substrateof plastic, ceramic or semiconductor material. Such coils conventionallycomprise up to four overlapping spiral layers, using currently availablefabrication techniques.

[0011] Photolithographic coils or antennas are also commonly used incontactless smart cards, as are known in the art. These cardsinductively communicate with and receive power from a reader circuitthrough a photolithographic coil or antenna embedded in the card.Because smart cards are limited in thickness to less than 0.8 mm, theygenerally include only a single coil, whose axis is necessarilyperpendicular to the plane of the card. To communicate with the reader,the smart card must be properly oriented, so that the coil axis isaligned with a magnetic field generated by the reader, in order toachieve proper coupling.

[0012] Reducing the width or outer diameter of the coil assembly wouldallow position determining systems to be used with narrower devices,which generally have superior maneuverability and ease of access toremote locations. Alternatively, reducing the width or outer diameter ofthe coil assembly would allow the assembly to occupy a smaller portionof the cross-sectional area of the device, leaving more space forfunctional apparatus and/or working channels along the devices.

[0013] To date, there have been no position sensors or sensor coilshaving outer diameters that are smaller in size than the sensorsdescribed above and are capable of achieving performance measures suchas maintaining a high degree of accuracy at high temperatures.

SUMMARY OF THE INVENTION

[0014] The present invention is directed toward a position sensor for amedical device comprising a core made of a Wiegand effect material and awinding circumferentially positioned around the core. The positionsensor is used to determine position and/or orientation coordinates.

[0015] The position sensor maintains accuracy within≦1 mm attemperatures greater than 75° C. Moreover, the position sensorpreferably maintains accuracy within<1 mm at temperatures atapproximately 80° C.

[0016] The core of the position sensor according to the presentinvention has an outer diameter of less than approximately 0.3 mm andpreferably the core has an outer diameter of about 0.25 mm.Additionally, in one embodiment, the winding is attached to the core.Moreover, a combination of the core and the winding have an outerdiameter of less than approximately 0.5 mm and preferably an outerdiameter of about 0.4 mm.

[0017] The core of the position sensor for one embodiment in accordancewith the present invention comprises cobalt, vanadium and iron.Moreover, the material of the core comprises approximately 20-80% cobaltin one embodiment. In another embodiment according to the presentinvention, the material of the core comprises approximately 2-20%vanadium. In another embodiment of the present invention, the materialof the core comprises approximately 25-50% iron. In a preferredembodiment according to the present invention, the material of the corecomprises approximately 52% cobalt, 10% vanadium and 38% iron.

[0018] In a preferred embodiment of the present invention, the positionsensor has accuracy to within approximately 0.5 mm. This type ofaccuracy is achieved with the position sensor in accordance with thepresent invention through the use of a position sensor having a coremade of a high permeable material wherein the material is a magneticmaterial that produces a magnetic field that switches polarity andcauses a substantially uniform voltage pulse upon an application of anexternal field.

[0019] In an alternative embodiment of the position sensor according tothe present invention, the material of the core comprises a copper,nickel, and iron alloy (CuNiFe). In another embodiment of the positionsensor according to the present invention, the material of the corecomprises an iron, chrome, and cobalt alloy. These alternativeembodiments for the core material also ensure accuracy withinapproximately 0.5 mm for the position sensor according to the presentinvention.

[0020] The present invention further includes a medical device andposition sensor combination comprising a medical device having a bodyand a position sensor attached to the body wherein the position sensorhas a core made of Wiegand effect material and a windingcircumferentially positioned around the core. The various embodimentsfor the position sensor outlined above are used in the medical deviceand position sensor combination in accordance with the presentinvention.

[0021] Both the position sensor and the medical device and positionsensor combination, both in accordance with the present invention, areused in conjunction with a position and orientation system thatmaintains accuracy even at high temperatures.

[0022] The present invention also includes a method for measuringtemperature at a site within a patient during a medical procedure. Themethod in accordance with the present invention comprises providing amedical device having a position sensor and placing the medical devicewithin the patient and positioning the position sensor at the site. Atemperature measurement signal is provided to the position sensor andvoltage is measured at the position sensor. A resistance value isdetermined based on the temperature measurement signal and the voltageand a temperature value based on the resistance value is thendetermined. The method for measuring temperature in accordance with thepresent invention utilizes the various embodiments for the positionsensor and medical device and position sensor combination as outlinedabove.

[0023] An algorithm stored in the memory of a signal processor utilizedby the position and orientation system is used to determine thetemperature at or adjacent the position sensor of the medical device.The algorithm further includes utilizing a resistance drift factor whichis added to the measured resistance value in accordance with thealgorithm according to the present invention.

[0024] The temperature measurement method in accordance with the presentinvention further comprises generating an externally applied field suchas an AC magnetic field at the site within the patient. The externallyapplied field is caused by a generator signal provided to a plurality ofmagnetic field generators. The temperature measurement signal is at adifferent frequency than the generator signal.

[0025] In one embodiment, the temperature measurement signal is at 4 KHzand the generator signal is at 3 KHz.

[0026] The present invention further includes a method for adjusting fortemperature sensitivity of a medical device having a position sensorwherein the method comprises the steps of providing a medical devicehaving a position sensor and measuring voltage at the position sensor. Aresistance value is determined from the measured voltage and atemperature value is determined at the position sensor based on theresistance value. Additionally, a sensitivity is determined at theposition sensor based on the temperature and location informationprovided by the position sensor is adjusted based on the sensitivity.

[0027] The method for adjusting for temperature sensitivity inaccordance with the present invention utilizes the position sensor andmedical device having position sensor combination as outlined above.

[0028] In accordance with the present invention, the locationinformation derived from the position sensor is in the form of positionand/or orientation coordinates. In accordance with the presentinvention, a sensitivity algorithm is stored in the memory of the signalprocessor of the position and orientation system. The temperaturesensitivity algorithm applies a resistance drift factor to theresistance value determined by the method for adjusting for temperaturesensitivity of the medical device having the position sensor inaccordance with the present invention.

[0029] Additionally, the sensitivity of the position sensor isdetermined by applying a sensitivity drift factor to the temperaturevalue determined above. The sensitivity drift factor is stored in thememory of a signal processor. Both the resistance drift factor and thesensitivity drift factor are derived from a resistance versustemperature profile of the position sensor and a sensitivity versustemperature profile of the position sensor respectively. Both theresistance versus temperature profile of the position sensor and thesensitivity versus temperature profile of the position sensor areprestored in the memory of the signal processor of the position andorientation system in accordance with the present invention.

[0030] These and other objects, features and advantages of the presentinvention will be more readily apparent from the detailed descriptionset forth below, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031]FIG. 1A is a view in cross-section of a sensor coil having a corefor use as a position sensor in accordance with the present invention;

[0032]FIG. 1B is a view in cross-section of the sensor coil of FIG. 1Aas a position sensor attached to a body of a medical device;

[0033]FIG. 2 is a schematic illustration of a testing apparatus for theposition sensor and medical device of FIGS. 1A and 1B according to thepresent invention;

[0034]FIG. 3A is a chart indicating a regressing heat experiment for theposition sensor according to the present invention plotting resistanceas a function of temperature;

[0035]FIG. 3B is a chart reflecting a regressing heat experiment for theposition sensor according to the present invention plotting sensitivityas a function of temperature;

[0036]FIG. 4 is a schematic illustration of another test systemutilizing the position sensor according to the present invention; and

[0037]FIG. 5 is a graph of the pulse output for the Wiegand effectmaterial of the position sensor according to the present inventionplotting voltage as a function of time.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0038] For purposes of this disclosure, the terms “sensor coil”, “coil”,“position sensor” and “location sensor” have the same meaning and areused interchangeably. A position sensor is a sensor that provideslocation information in the form of signals that determine positionand/or orientation coordinates of the position sensor in the mannerdescribed above.

[0039] The present invention, as best illustrated in FIGS. 1A and 1B,show a position sensor according to the present invention comprising asensor coil 10 having a core 12 made of Wiegand effect material, whichis described in greater detail below, and a winding (in the form ofcopper wire) attached to or circumferentially wrapped around the core12. The sensor coil 10 is particularly useful as a position sensor for amedical device 80 as shown in FIG. 1B. As mentioned previously, thesensor coil 10 is used as a position sensor for determining locationinformation in the form of position coordinate and/or orientationcoordinates.

[0040] The sensor coil 10 preferably has a length L of approximately3.0-4.0 mm although the length L can be longer. Wires 16 are connectedto the sensor coil 10 leading from the winding wire 14 wherein the wires16 are operatively connected to a circuit for measurement of the voltageinduced in the sensor coil 10.

[0041] As shown in FIG. 1B, the core 12 of the sensor coil 10 has anouter diameter (OD₁) less than 0.3 mm and preferably an OD₁ of about0.25 mm. The total outer diameter for the sensor coil 10 (OD₂) is lessthan 0.5 mm and preferably an OD₂ of about 0.4 mm. Due to the extremelycompact size of the sensor coil 10 according to the present invention,the sensor coil 10 can be accommodated in the body 85 of the medicaldevice 80 wherein the medical device 80 has an outer diameter (OD₃) lessthan or equal to approximately 0.67 mm (2F or less). Thus, sensor coil10 is useful as a position sensor in various medical devices 80. Forinstance, the medical device 80 preferably includes devices such as acatheter, a probe, a stent, a tag or marker, etc. At these dimensions,the medical device 80 including the sensor coil 10 according to thepresent invention is utilized in various medical applications such asdiagnostic and/or therapeutic procedures performed in the various tissueand organs of a patient's body.

[0042] The sensor coil 10 according to the present invention isparticularly useful for medical devices 80 using a single sensor coil asa position sensor although it can be utilized in position sensors havingmultiple sensor coil arrangements such as three mutually orthogonalsensor coils. Medical devices 80 using an arrangement of only one sensorcoil 10 are referred to as a “single axis system”. For the sensor coil10 according to the present invention, the sensor coil 10 has a length Lthat is at least two to three times the outer diameter OD₂ of the sensorcoil 10 and preferably a length L that is greater than six times the OD₂of the sensor coil 10. Thus, the sensor coil 10 according to the presentinvention is more sensitive than the prior art sensor coils havingferrite or carbonyl iron cores. Moreover, the length/OD ratio of thesensor coil 10 ensures that the sensor coil 10 is easier and cheaper tomanufacture since it is mechanically more stable in comparison withsensor coils having ferrite material with a similar length/OD ratiowhich would tend to result in a sensor core that is brittle.

[0043] Core Material

[0044] In accordance with the present invention, the material for thecore 12 is a material of high permeability and high mechanicalflexibility such as Wiegand effect material, which may be in the form ofa wire. Wiegand effect material is usually produced by cold working a0.010 inch diameter ferromagnetic wire. The wire is made from Vicalloywhich is a mixture of cobalt, iron, and vanadium (manufactured by HIDCorporation of North Haven, Conn., USA). This material is a speciallywork hardened, self-nucleating bi-stable magnetic material, which can bein the form of a wire, and can generate pulses up to 600 millivoltswithout any electrical inputs. It works by control of the Barkhausenjumps. For purposes of this disclosure, the terms “Wiegand effectmaterial”, “Wiegand material, “Wiegand alloy”, and “Wiegand wire” havethe same meaning and are used interchangeably.

[0045] With respect to the use of this Wiegand effect material for thecore 12 of the sensor coil 10, the material comprises various mixturecombinations of cobalt, vanadium and iron respectively. For instance, inone embodiment for the sensor coil 10, the core material comprisesapproximately 20%-80% cobalt and the remaining percentage of thematerial comprises vanadium and iron. In another embodiment for thesensor coil 10, the core material comprises approximately between 2%-20%vanadium and the remaining percentage of the material comprises cobaltand iron. In another embodiment of the sensor coil 10, the core materialcomprises approximately between 25%-50% iron and the remaining percentof the material comprises cobalt and vanadium.

[0046] In a preferred embodiment for the core 12 of the sensor coil 10,the core material comprises approximately 52% cobalt, 10% vanadium and38% iron. It is important to note that core material for the core 12 maycomprise any desired combination and percentage of composition inaddition to those combinations illustrated above.

[0047] The cold working process utilized on the Wiegand effect materialconsists of several steps of increasing amounts of twist and detwist ofthe Wiegand material (wire) under applied tension.

[0048] The Wiegand effect wire is then age hardened to hold in thetension built up during the cold working process. This process causesthe Wiegand effect material to have a soft magnetic center and a workhardened surface, which has a higher magnetic coercivity, called the“shell”.

[0049] When an alternating magnetic field of proper strength is appliedto the Wiegand material, the magnetic field of the material centerswitches polarity and then reverses, causing a sharp, substantiallyuniform voltage pulse to be generated wherein the pulse is commonlyreferred to as a “Wiegand Pulse”. The cold working process to produceWiegand material permanently “locks” the ability to exhibit the wellknown Barkhausen jump discontinuities into the material.

[0050] With the Wiegand effect material as a core material, magneticswitching occurs when the Wiegand material is in the presence ofalternating longitudinal magnetic fields. Since the resultant hysteresisloop contains large discontinuous jumps known as Barkhausendiscontinuities which occur due to shell and center polarity switching.The magnetic switching action of the Wiegand material induces a voltageacross the pick-up coil windings 14 of approximately 10 microseconds induration as shown in FIG. 5.

[0051] With the Wiegand material, the pulse amplitude is not totallydependent on excitation field strength and orientation. The alternatingpositive and negative magnetic fields of equal saturating strength areused to magnetize and trigger the Wiegand material when in use for theposition sensor 10. These alternating magnetic fields are produced by analternating current generated field.

[0052] Moreover, the Wiegand effect is operational at temperaturesranging between −80° C. to 260° C. Thus, functional temperature range ofeach position sensor 10 is based on the limitations of various componentsubparts of the individual sensor.

[0053] Additionally, in an alternative embodiment of the presentinvention, the core 12 of the sensor coil 10 consists of an alloymaterial comprising a mixture of copper, nickel and iron (CuNiFe).Alternatively, another embodiment of the present invention uses a core12 consisting of an alloy material comprising a mixture of iron, chromeand cobalt, for instance, ARNOKROME™ manufactured by the Rolled ProductsDivision of the Arnold Engineering Company (SPC Technologies, Marengo,Ill., USA). Both of these materials, e.g. CuNiFe and iron, chrome andcobalt alloys are also highly permeable and highly mechanically flexiblematerials and are utilized as core material 12 for the sensor coil 10 inaccordance with the present invention.

[0054] Temperature Sensitivity Testing

[0055] In accordance with the present invention, temperature sensitivitytesting was conducted on the position sensor 10 for the development of atemperature sensitivity algorithm (described in detail below) particularto a location system 30 (FIG. 4) having one sensor coil 10 as theposition sensor on the medical device 80, particularly, the single axissystem described in commonly assigned U.S. patent application Ser. No.09/620,316, filed on Jul. 20, 2000 which disclosure is incorporatedherein by reference. Accordingly, the temperature sensitivity algorithmis used in conjunction with the position and orientation algorithm ofthe location system 30 (FIG. 4), for instance a single axis locationsystem, in order to compensate for changes in temperature sensitivityfor utilizing the position sensor 10 at high temperatures whilemaintaining a high degree of accuracy, for instance, ≦1 mm andpreferably ≦0.5 mm in accordance with the present invention.

[0056] In creating the temperature sensitivity algorithm according tothe present invention, heat regression tests were conducted in order totest the resistance and sensitivity for the sensor coils 10 having cores12 made of Wiegand effect material as a function of temperature as bestshown in Appendix Table 1 and FIGS. 3A and 3B. These tests establishedvalues and ranges particular to the position sensor 10 of the presentinvention. For instance, these predetermined values include a resistancedrift value (G_(r)) over a large temperature range (30° to 80° C.) forthe sensor resistance versus temperature results shown in FIG. 3A andAppendix Table 1 and a sensitivity drift value (G_(s)) over the same 30°to 80° C. temperature range for the sensor sensitivity versustemperature results shown in FIG. 3B and Appendix Table 1.

[0057] These values were predetermined by testing the effect of thesensor core 12 composition (Wiegand effect material) on the temperaturesensitivityfor twenty sensors 10 (data from eight position sensors 10representative of all twenty sensors 10 tested are listed in AppendixTable 1). For this test, each location sensor 10 consisted of a singlesensor coil 10 having core 12 made of Weigand effect material. Thetemperature sensitivity for each of the sensor coils 10,as the positionsensor, were tested in an apparatus as schematically shown in FIG. 2.Accordingly, position sensor (sensor coil 10) and thermocouple 22 wereinserted into a glass tube 24, which was, in turn, placed in a hot waterbath 26. Each sensor coil 10 and thermocouple 22 have wire leads 36 and38, respectively which are attached to instruments to measure sensorvoltage and temperature, respectively. Water was poured into the bath toa level sufficient to submerge each sensor 10. The bath was placedinside a Helmholtz chamber, consisting of three pairs of mutuallyorthogonal Helmholtz coils.

[0058]FIG. 2 shows two of the three pairs of Helmholtz coils; the firstpair consisting of Helmholtz coils 28 and 30; and the second pairconsisting of Helmholtz coils 32 and 34. The Helmholtz coils arearranged such that the distance between the Helmholtz coils in a pair isequal to the radius of each of the Helmholtz coils in the pair. In theHelmholtz chamber, each pair of Helmholtz coils is disposed coaxially,the three pairs of Helmholtz coils having three, mutually orthogonalaxes. The Helmholtz chamber has the property that the magnetic fieldwithin the chamber is relatively invariant with distance from the centerof the chamber. Nevertheless, in testing the position sensors 10,efforts were made to locate the sensors 10 in the same spot within thechamber. The Helmholtz coils were energized with alternating current(AC) having a frequency of 3 KHz. Sensor voltages were measured from onesensor coil 10 in each position sensor in five degree increments overthe temperature range of 30° to 80° C. Measurements were performed onthe twenty position sensors 10 wherein the sensor coil cores 12 were allmade of Wiegand effect material for determining parameters such asresistance drift G_(r), temperature sensitivity drift G_(s), resistancedrift versus temperature slope a₀ and sensitivity drift versustemperature slope b₀ used to establish a sensitivity correction S(T),e.g. as part of a real time sensitivity compensation algorithm for theposition and orientation algorithm of the location system 30.

[0059] A 4 KHz signal is sent through the sensor coil 10 and the voltageacross the coil 10 is measured. The ratio of voltage and the 4 KHzcurrent (I) is the resistance. The 4 KHz signal is used in order not todisturb the other frequencies of the system 30 which are below 4 KHz.

[0060] First, since the current (I) delivered through each sensor 10 isconsistent and uniform (from a 4 KHz signal delivered by the positionand orientation system 30 shown in FIG. 4),the voltages read on eachsensor 10 at each temperature were converted to a resistance value bysignal processor 48. Resistance drift values G_(r) were plotted againsttemperature as illustrated in FIG. 3A and Appendix Table 1. Theresistance values (in ohms) were measured at each temperature along theselected temperature range (30° to 80° C.) and were converted to thegradient values G_(r) (as % drift of resistance), i.e. the percentagedifference of a sensor coil resistance at temperature T relative to itsresistance at 80° C. according to the equation:${{Gr}(\%)} = {\frac{{{R(T)} - {R(80)}}}{R(80)} \times 100}$

[0061] wherein G_(r) is resistance drift as the gradient value inpercent (% drift), R(T) is the sensor coil resistance at temperature Tand R(80) is the sensor coil resistance at 80° C. Based on theseresults, the total resistance drift was approximately 13% over theentire temperature range. As shown in FIG. 3A, the plot shows a linearrelationship (linear curve) between resistance drift and temperature andthe slope b₀ for this resistance change is relatively constant atapproximately 0.30%/degree for all of the sensors 10 tested.Accordingly, a resistance drift factor b₀, e.g. 0.30 (slope of theresistance versus temperature data), is predetermined, set and stored inthe signal processor 48 for the location system 30.

[0062] Additionally, sensitivity (S) in V/gauss at KHz was also measuredfor each of the sensors 10 over the temperature range and sensitivitydrift G_(s) was determined and plotted against temperature as shown inFIG. 3B. These sensitivity measurements S were converted to gradientvalues G_(s) (as % drift of temperature sensitivity), i.e., thepercentage difference of a sensor coil voltage at temperature T relativeto its voltage at 80° C., according to the following equation:${{Gs}(\%)} = {\frac{{{V(T)} - {V(80)}}}{V(80)} \times 100}$

[0063] wherein G_(s) is sensitivity drift as the gradient value inpercent (% drift), V(T) is the coil voltage at temperature T and V(80)is the sensor coil voltage at 80° C. Based on these results, the totalsensitivity drift is approximately 1.24% over the entire temperaturerange and the slope a₀ for this sensitivity drift versus temperatureprofile is approximately 0.025%/degree. Thus, a sensitivity drift factora₀, e.g. the 0.25 slope, is a constant that is predetermined, set andstored in the signal processor 48 for the location system 30.

[0064] Temperature Sensitivity Algorithm and Use

[0065] Based on the testing conducted, a temperature sensitivityalgorithm has been created for the location system 30. The data from thetesting showed that the resistance change b₀ and the sensitivity changea₀ for the position sensors 10 tested are constants as evidenced by theresults in Appendix Table 1 and FIGS. 3A and 3B. Both of these constants(a₀ and b₀) are stored in the memory of the signal processor 48 for thelocation system 30.

[0066] Additionally, each position sensor 10 (used on the medical device80) is calibrated at room temperature, for instance, temperature rangebetween 20°-23° C. in order to set an initial sensitivity SO and aninitial resistance Ro for each position sensor 10. These values are alsostored in the signal processor 48 in the memory portion, for instance,the EPROM.

[0067] When in use, the medical device 80 having the position sensor 10is placed within a patient and within an externally applied generated ACmagnetic field from a plurality of magnetic field generators (not shown)positioned external to the patient. When using the medical device 80,for instance in a procedure such as an ablation procedure, current (I)is delivered through the position sensor 10 as a consistent and uniformsignal, for instance, a 4 KHz signal delivered by the location system30. The voltage value is determined at the sensor 10 and the voltagevalue is converted to the resistance value R(T) by signal processor 48according to the formula R(T)=V/I. In turn, the real time temperature(T) at the position sensor 10 is determined according to the formula:$T = \frac{{R(T)} - R_{o}}{b_{0}}$

[0068] where R(T) is the resistance determined at the current or realtime temperature at the position sensor 10, R₀ is the initial resistancedetermined during the calibration procedure and recalled from the signalprocessor memory and b₀ is the resistance drift factor also recalledfrom memory.

[0069] The next step after calculating the real time temperature T is todetermine the current or real time sensitivity S(T) of the positionsensor 10 at this temperature according to the formula:

S(T)=S _(o) +a _(o) XT

[0070] where S₀ is the initial sensitivity for the position sensor 10,a₀ is the sensitivity drift factor (both determined during thecalibration procedure and recalled from memory), and T is the real timetemperature calculated above.

[0071] In the next step, the position and orientation algorithm(location algorithm) of the location system 30 is adjusted in order toaccount for the real time sensitivity S(T) which is now used as acorrection factor for the position and orientation algorithm accordingto the formula: $B = \frac{V}{S(T)}$

[0072] where B is the calculated magnetic field at the measured at theposition sensor 10, V is the voltage at the position sensor 10 and S(T)is the real time sensitivity of the position sensor 10 at the real timetemperature. In turn, the new magnetic field measurement B is used inthe position and orientation algorithm to calculate the location, e.g.the position and orientation, of the position sensor 10.

[0073] Accordingly, at any given moment during use of the medical device80 and the location system 30, the accuracy of the position andorientation coordinate information derived from the position sensor 10is maintained to an accuracy of ≦1 mm and preferably ≦0.5 mm through useof the temperature sensitivity algorithm in accordance with presentinvention.

[0074] Accuracy Testing

[0075] Additionally, another test was performed to measure the effect ofsensor coil core composition on the determined location of a medicaldevice 80 under simulated ablation conditions (high temperature) usingthe apparatus, including the location system 30, schematically shown inFIG. 4. The distal tips of medical devices, e.g. an ablation catheter 80and a reference catheter 80 a were securely fastened in water bath 44 toprevent movement of the catheter tips during the test. The ablationcatheter 80 and reference catheter 80 a both contained position sensors.In addition, the ablation catheter 80 was equipped with a 4 mm longablation electrode 91 at its distal tip. The bath was filled with saltwater having an impedance of about 100 ohms to simulate blood. Theproximal ends of the catheters 80 and 80 a were connected to junctionbox 46 through which electrical signals from and to the position sensorsand electrode 91 could be received and transmitted. Junction box 46 wasconnected to signal processor 48 to compute the location (in positionand orientation coordinate form) of the ablation catheter tip 80relative to the reference catheter tip 80 a. RF generator 50 wasconnected to junction box 46 to supply RF energy to the ablationelectrode 91 at the distal tip of ablation catheter 80. RF generatorreturn electrode 52 was also contained in bath 44 and was connected toRF generator 50.

[0076] The apparatus of FIG. 4 was contained within a magnetic fieldgenerated by three magnetic field generator elements, e.g.electromagnets (not shown) arranged in a triangular arrangement roughly40 cm between centers positioned below the apparatus. For each cathetertested, ten location readings were made prior to supplying RF energy tothe catheter tip electrode 91. Another ten location readings were madeafter the supply of RF energy was initiated to the distal tip electrodeat a power level of 70 W. Several types of catheters were evaluated. Thecatheter types included catheters having location sensors having sensorcoil cores comprising ferrite; catheters having location sensors withsensor coil cores comprising carbonyl iron and in accordance with thepresent invention, catheters 80 having sensor coils 10 with cores 12made of Wiegand effect material. The temperature sensitivity correctionalgorithm was employed by the signal processor 48 when testing thecatheters 80 having the sensor coils 10 with Weigand effect materialcores 12 of the present invetion.

[0077] Since the catheter tips were securely fastened to the bath duringthe tests, absent any location error, the catheter tips should haveregistered the same location before and during the supply of RF energyto the distal tip electrodes. In fact, differences between tip locationbefore and during supply of RF energy were observed. The averagelocation error of the catheters during simulated ablation conditions(defined as the absolute value of the difference in tip location beforeand during supply of RF energy to the catheter tip electrode) as afunction of sensor core composition is shown in Table 2 below. TABLE 2LOCATION ERROR (MM) AND SENSITIVITY (V/GAUSS AT 3 KHZ) AS A FUNCTION OFSENSOR COIL CORE COMPOSITION SENSOR COIL CORE AVERAGE LOCATIONSENSITIVITY COMPOSITION ERROR (mm) (V/GAUSS) Ferrite 5.9 3.0 CarbonylIron 0.4 3.3 Wiegand Alloy 0.5 7.0-8.0

[0078] As shown in Table 2, although the sensor coil 10 with the Wiegandeffect material (Wiegand alloy) core 12 demonstrated a greater than 2×increase in sensitivity over both the ferrite and carbonyl iron coresensor coils, a high degree of accuracy was still maintained, e.g. only0.5 mm error. This minimal error in the position and orientationcoordinate information was a direct result of the temperaturesensitivity correction algorithm according to the present invention.Accordingly, even though the position sensor 10 according to the presentinvention reflected an overall sensitivity ranging from between 7.0-8.0V/gauss, the position sensor 10 demonstrated a high degree of accuracydue to its minimal location error. Thus, the position sensor 10according to the present invention is particularly useful for variousmedical applications including even those medical applications thatexperience high temperatures up to 80° C. such as thermal ablationprocedures. Moreover, the average location error of approximately 0.5 mmfor the position sensor 10 of the present invention and this isextremely close, if not negligible, to the location error observed withthe carbonyl iron core sensor coils even though these coils demonstratelower sensitivity. Thus, there is a negligible drop-off in accuracy inexchange for the significant decrease in size afforded by the positionsensor 10 of the present invention. A size benefit that cannot beachieved with position sensors utilizing either ferrite or carbonyl ironas its core material because of challenges posed by the handling andmanufacturing requirements of these two materials. The drawbacksassociated with these two materials are due to characteristics such asbrittleness in these materials which have an overall limitation on theratio between length and diameter for the sensor. Accordingly, since theposition sensor 10 of the present invention eliminates these drawbacks,it can be utilized in much smaller sized devices, such as sizes outlinedabove, than previuosly thought possible.

[0079] Temperature Measurement with Position Sensor

[0080] The present invention also includes a method for measuring thetemperature adjacent the position sensor (sensor coil) 10 on the medicaldevice 80 utilizing the sensor coil 10 and the position and orientationsystem 30. The method of measuring temperature, at the medical device80, in accordance with the present invention includes establishing atemperature measurement signal distinct from the signal used to energizethe electromagnetic field generators (not shown) of the system 30. Aswith the field generator signal, the temperature measurement signal isan AC signal. However, the temperature measurement signal is at adifferent frequency from the frequency used to drive the fieldgenerators.

[0081] The temperature measurement signal is a uniform AC signaltransmitted to the sensor coil 10 by the system 30. For instance, thefield generators are driven by a field generator signal having afrequency of 3 KHz and the temperature measurement signal sent to thesensor coil 10 has a frequency of 4 KHz.

[0082] As the medical device 80 is used in a medical procedure, forinstance an ablation procedure, the temperature generated during theprocedure by the device 80 or other devices which may be utilized alongwith the device 80 are monitored and measured by the system 30 using thesensor coil 10 of the device 80. As mentioned above, the methodaccording to the present invention is particularly useful for directmeasurement of temperatures adjacent the sensor coil 10 of the device80.

[0083] In measuring temperature, the temperature measurement signal, forinstance a 4 KHz signal, is provided to the sensor coil 10 and thevoltage across the sensor coil 10 is measured by the system 30 throughthe signal processor 48. Since the temperature measurement signal, e.g.current (I) and the measured voltage are both known at this point, thesignal processor 48 readily determines the resistance at the sensor coil10 based on these two values.

[0084] In accordance with the temperature sensitivity algorithm of thepresent invention outlined above, the resistance value determined by thesignal processor 48 (based on the temperature measurement signal and themeasured voltage) is adjusted by the resistance drift factor b₀constant) by the signal processor 48. Accordingly, this adjustmentallows the signal processor 48 to accurately determine the actualtemperature at the sensor coil 10.

[0085] Accordingly, in a temperature measurement method in accordancewith the present invention, the medical device 80 having sensor coil 10is placed within a patient and within a magnetic field at a desired sitefor performing a medical procedure with the device 80. The position andorientation system 30 generates a magnetic field through a generatorsignal provided to the plurality of magnetic field generators (notshown). As mentioned above, a field generator signal at a firstfrequency, for instance 3 KHz is provided to the field generators and atemperature measurement signal (I) at a second frequency, for instance 4KHz, is provided to the sensor coil 10.

[0086] As the medical device 80 is being used at the desired site withinthe patient and within the externally applied magnetic field, a voltagemeasurement is made by the signal processor 48 in order to measure thevoltage across the sensor coil 10. In accordance with the algorithmdescribed above, both the temperature measurement signal (I) and themeasured voltage value are used by the signal processor 48 to determinea resistance value at the sensor coil 10. And, in accordance with thisalgorithm, an actual temperature value is determined in real time basedon the actual temperature measured with the sensor coil 10.

[0087] Thus, with the actual temperature value, the operator orphysician utilizing the system 30 can take appropriate actions. Forinstance, if the temperature generated during a procedure, such as anablation procedure, becomes too high, for instance, exceeds 80° C., thephysician may want to pause the procedure and allow for the temperatureto cool at the site prior to continuing with the procedure. This is adirect safety benefit to the patient.

[0088] Accordingly, the temperature measurement method according to thepresent invention provides the physician with great flexibility andavoids having to use separate temperature monitors or temperaturesensors such as thermocouples. Thus, by utilizing the sensor coil 10 inaccordance with the present invention, the overall costs of the medicalprocedure are also reduced.

[0089] While preferred embodiments of the present invention have beenshown and described herein, it will be obvious to those skilled in theart that such embodiments are provided by way of example only. Numerousvariations, changes, and substitutions will now occur to those skilledin the art without departing from the invention. Accordingly, it isintended that the invention be limited only by the spirit and scope ofthe appended claims.

What is claimed is:
 1. A position sensor for a medical device, theposition sensor comprising: a core made of a Wiegand effect material;and a winding circumferentially positioned around the core.
 2. Theposition sensor according to claim 1, wherein the position sensor isused to determine position coordinates.
 3. The position sensor accordingto claim 2, wherein the position sensor is also used to determineorientation coordinates.
 4. The position sensor according to claim 1,wherein the position sensor maintains accuracy of ≦1 mm at temperaturesgreater than 75° C.
 5. The position sensor according to claim 4, whereinthe position sensor maintains accuracy of ≦1 mm at temperatures atapproximately 80° C.
 6. The position sensor according to claim 1,wherein the core has an outer diameter less than approximately 0.3 mm.7. The position sensor according to claim 6, wherein the core has anouter diameter of about 0.25 mm.
 8. The position sensor according toclaim 7, wherein the winding is attached to the core.
 9. The positionsensor according to claim 8, wherein a combination of the core and thewinding has an outer diameter less than approximately 0.5 mm.
 10. Theposition sensor according to claim 9, wherein the combination of thecore and the winding have an outer diameter of about 0.4 mm.
 11. Theposition sensor according to claim 10, wherein the material of the corecomprisescobalt
 12. The position sensor according to claim 11, whereinthe material of the core further comprises vanadium.
 13. The positionsensor according to claim 12, wherein the material of the core furthercomprises iron.
 14. The position sensor according to claim 13, whereinthe material of the core comprises approximately 20%-80% cobalt.
 15. Theposition sensor according to claim 13, wherein the material of the corecomprises approximately 2%-20% vanadium.
 16. The position sensoraccording to claim 13, wherein the material of the core comprisesapproximately 25%-50% iron.
 17. The position sensor according to claim13, wherein the material of the core comprises approximately 52% cobalt,10% vanadium and 38% iron.
 18. The position sensor according to claim 8,wherein the winding is made of copper.
 19. The position sensor accordingto claim 3, wherein the position sensor has an accuracy withinapproximately 0.5 mm.
 20. A position sensor for a medical device, theposition sensor comprising: a core made of a high permeable material,the material being a magnetic material that produces a magnetic fieldthat switches polarity and causes a substantially uniform voltage pulseupon an application of an external field.
 21. The position sensoraccording to claim 20, further comprising a winding circumferentiallypositioned around the core.
 22. The position sensor according to claim20, wherein the position sensor is used to determine positioncoordinates.
 23. The position sensor according to claim 22, wherein theposition sensor is also used to determine orientation coordinates. 24.The position sensor according to claim 20, wherein the position sensormaintains accuracy of ≦1 mm at temperatures greater than 75° C.
 25. Theposition sensor according to claim 24, wherein the position sensormaintains accuracy of ≦1 mm at temperatures at approximately 80° C. 26.The position sensor according to claim 20, wherein the core has an outerdiameter less than approximately 0.3 mm.
 27. The position sensoraccording to claim 26, wherein the core has an outer diameter of about0.25 mm.
 28. The position sensor according to claim 27, wherein thewinding is attached to the core.
 29. The position sensor according toclaim 28, wherein a combination of the core and the winding has an outerdiameter less than approximately 0.5 mm.
 30. The position sensoraccording to claim 29, wherein the combination of the core and thewinding have an outer diameter of about 0.4 mm.
 31. The position sensoraccording to claim 30, wherein the material of the core comprisescobalt.
 32. The position sensor according to claim 31, wherein thematerial of the core further comprises vanadium.
 33. The position sensoraccording to claim 32, wherein the material of the core furthercomprises iron.
 34. The position sensor according to claim 33, whereinthe material of the core comprises approximately 20%-80% cobalt.
 35. Theposition sensor according to claim 33, wherein the material of the corecomprises approximately 2%-20% vanadium.
 36. The position sensoraccording to claim 33, wherein the material of the core comprisesapproximately 25%-50% iron.
 37. The position sensor according to claim33, wherein the material of the core comprises approximately 52% cobalt,10% vanadium and 38% iron.
 38. The position sensor according to claim28, wherein the winding is made of copper.
 39. The position sensoraccording to claim 23, wherein the position sensor has an accuracywithin approximately 0.5 mm.
 40. The position sensor according to claim20, wherein the material of the core comprises a copper, nickel and ironalloy (CuNiFe).
 41. The position sensor according to claim 20, whereinthe material of the core comprises an iron, chrome and cobalt alloy.